POWER GENERATION AND TRANSMISSION 23 r £21 13 Cuf-off, Proportion of Stroke (for 1,3) 0.1 02 0.3 04 05 0.6 0:7 0.8- 0.9-''Ift 1JL 3P 40 .^ 50 60 70 80 90 100 110 120 130 Ffer Cent of Rating (for Z.^&.S,?) FIG. 17.—Ratio of expansion and steam consumption. Variables Affecting Economy.—Steam consumption is in practice reduced by high pressure, superheat, a mean value1 for the ratio of expansion, good vacuum or low back pressure, multiple expansion (compound engine), low clearance, tight valves and pistons, good bearings and lubrication, and a valve gear which can be adjusted in each of its functions independently, while giving quick opening and closure. Wetness of steam has little influence, although mechanically objectionable. An engine need not be large to be economical. Some of the best records have been.made by 100-hp. units. High speeds decrease cylinder condensation but limit the choice of a valve gear. They also require high compression, which is not economical. Every engine has a load of best economy (usually the rated load). If the load is variable, a flat characteristic (steam rate plotted against load or per cent of rating) is desirable. Pressures are now rarely below 100 lb.: 150 Ib. is common, large central stations usually approach 200 lb. and 300 lb. is being introduced occasionally. Superheat, if used, should be at least 100°. STEAM TURBINES Types.—Practically all turbines are now built with horizontal shafts. The direction of steam flow (axial, vertical or tangential) is a matter of individual characteristic: axial flow is generally used in large machines. The turbine utilizes the velocity of steam. The velocity may be produced by one sudden expansion through a nozzle. This gives the simple impulse type, in which velocities (condensing) range upward of 4,000 feet per second. Alternately, the steam may expand (decrease in pressure) while traversing the buckets, the velocity being augmented by expansion a little more rapidly than it is expended in doing work and overcoming friction. This gives the pressure or reaction type. Impulse turbines use reaction in part, and pressure turbines employ impulse to some extent. In the pressure turbine, the velocities seldom rise above 900 feet per second. Hence the annulus filled by the blades must be large: i.e., pressure turbines are large machines, usually consisting of two or more drums, each of which necessarily carries a rather large number (15 or more) rows of buckets. Compounding.—For maximum efficiency, the peripheral speed of the wheel at the bucket pitch line should approach the value of one-half the steam speed. In simple impulse turbines, this leads "either to very high rotative speeds or very large diameters. If the peripheral speed is reduced one-half, the efficiency decreases about one-third. Small capacity turbines of this type must therefore have ridiculously large wheels or else run very fast. Thus the original De Laval turbines used speeds of 10,000 to 30,000 r.p.m., with reduction gears. The present De Laval simple wheel uses peripheral speeds up to 1,300 ft. per second, implying 24,900 r.p.m. at 1 ft. diameter. 1 Cylinder condensation makes maximum values undesirable, in spite of the superior ideal economy due thereto.